This invention relates to an instrument and to the use of that instrument to determine certain thermophysical properties of solid samples. In particular, it relates to an instrument having an elliptical mirror, a light source at one focus, and a beam guide at the other focus, where the beam guide conducts light into a furnace containing the samples to be analyzed.
Thermal diffusivity is the speed that heat travels through a material. In the flash method of measuring thermal diffusivity, an energy pulse is deposited on the front face of a slab of uniform thickness and the resulting temperature rise on the back face is recorded as a function of time. By assuming a slab of homogenous solid material and a uniformly distributed, infinitesimally short duration energy pulse, Parker, et al. (J.Appl. Phys., 32(9): 1679-1684, 1961) were able to derive the thermal diffusivity, xcex1, from that temperature time relationship using the equation   a  =            0.138      ⁢              d        2                            π        2            ⁢              t                  1          2                    
where d is the thickness of the slab (in millimeters) and txc2xd is the time (in seconds) required for the temperature of the back face to reach one half of its maximum value (the xe2x80x9chalf-max timexe2x80x9d). The simple elegance of this relationship has made the method very popular and instruments based on it are commercially available.
Since thermal diffusivity can now be more easily determined, it is also easier to determine other thermal properties that are related to it through the fundamental equation K=xcex1xcfx81Cp, K is the thermal conductivity (W/mK), Cp is the specific heat capacity (joules/kgK), and xcfx81 is bulk density (kg/m3). Both Cp and K are very important in design work, but are often more difficult to measure, while bulk density xcfx81, and now thermal diffusivity xcex1, can be found more readily. Thus, if either the Cp or the K of a material could be determined by experiment, the other property could be calculated.
In theory, one can determine the heat delivered to a sample (Q, in joules) by an energy pulse then measure the increase in the sample""s absolute temperature (xcex94T). Assuming adiabatic conditions, specific heat capacity (Cp) can then be computed from the equation Cp=Q/mxcex94T, where m is the mass of the sample in kilograms. In practice, the heat actually absorbed by a sample cannot be determined with any degree of certainty. It is therefore necessary to use a less direct method of determining heat capacity.
Limited precision can be achieved by testing a sample of known heat capacity, then a sample of unknown heat capacity. If the heat loss for both samples is the same and the energy pulse source does not vary between the tests, the ratio of the maximum temperature increases of the two samples will be equal to the ratio of their respective heat capacities. The above assumptions, however, are a serious limitation on the accuracy of the process. Inaccuracies stem mainly from the fact that there is an appreciable time interval in between measurements because each sample is separately heated to the test temperature, allowed to equilibrate at that temperature, then tested. When there is a long period of time between measurements, it is extremely difficult to provide an energy pulse that is the same and measure the small temperature increases that are due to that energy pulse above the large background noise signals emitted by the furnace environment. As a result, current techniques usually have a scatter of data of about xc2x110%
We have invented an instrument for determining certain thermophysical properties of solid samples without measuring or determining the energy absorbed by a sample. In our instrument, a light source is placed at one focus of an elliptical mirror. At the other focus is placed a beam guide that conducts light from the light source into a furnace where it heats samples. At least two samples are heated to the testing temperature at the same time in the furnace, thereby assuring that they are at the same test temperature and eliminating the time required to heat each sample to that temperature by itself A simple light source provides the energy pulse and the light is concentrated by the elliptical mirror before it enters the beam guide. The instrument can hold both diffusivity samples and expansivity samples in the same environment, so that both diffusivity and in-situ density determinations derived from expansivity measurements can be made on the same sample material at the same time.